Legal as well as commercial demands are driving a great interest in developing methods for positioning mobile stations (MS's). While mobile positioning may be achieved through the use of external systems, like Global Positioning Satellite (GPS) systems, mobile positioning can also be achieved by means internal to the cellular system. Several cellular positioning methods have been proposed, each of which relies upon measurements such as time of arrival (TOA), round trip delay, or angle of arrival of communication signals. Cellular positioning methods can be divided into uplink or downlink methods, i.e., whether the base station (BS) performs measurements on mobile station originating signals (uplink methods) or the mobile station performs measurements on base station originating signals (downlink methods). One example of a proposed downlink method is the Enhanced-Observed Time Difference (E-OTD) method. The E-OTD method is based on measuring the time-of-arrival (TOA) of bursts transmitted from a BS on its broadcast control channel (BCCH) carrier.
All location or positioning methods require knowledge of a number of parameters, some time-varying and others of more or less constant nature. These parameters can be TOA measurements, angle of arrival measurements, signal strength measurement or other parameters known to be used in mobile station positioning. For example, the E-OTD method requires that the location of the base stations and the real time difference (RTD) between transmissions from pairs of base stations are precisely known. Whenever these conditions are not fulfilled the location accuracy can be degraded.
As discussed above, the Enhanced-Observed Time Difference is one example of a downlink positioning method. The E-OTD method is based on three quantities: the observed time difference (OTD); the real time difference (RTD); and the geometric time difference (GTD). These three quantities are related by the basic E-OTD equation:OTD=RTD+GTD  (1)where OTD is the time difference between the reception of signals from two base stations, RTD is the timing or synchronization difference between two base stations, and GTD is the difference of propagation path length between two base stations. FIG. 1 depicts these quantities which are used in the E-OTD method.
Referring to FIG. 1, if bursts from base station BS1 (e.g. serving base station) are received by a mobile station MS1 at time t1 and bursts from a neighbor base station BS2 are received by the mobile station at time t2, then the observed time difference is calculated asOTD=t2−t1.  (2)Likewise, if BS1 transmits its bursts at a time t3 and BS2 transmits its bursts at time t4, then the real time difference is calculated asRTD=t4−t3.  (3)Finally, if d1 is the propagation path length from BS1 to MS1, and d2 is the propagation path length between BS2 and MS1, then the geometric time difference is calculated asGTD=(d2−d1)/c,  (4)where d1 and d2 are the distance of the propagation path from BS1 and BS2, respectively, and c is the speed of the radio waves, usually taken as the speed of light in vacuum.
As illustrated in equation 5 below, if the OTD and RTD is known, the GTD can be determined in order to derive the location (i.e., position) information.GTD=OTD−RTD  (5)Since the GTD is based on the difference in distance between a mobile station and two base stations, a known GTD defines a hyperbola upon which the Mobile station can be located. The position of the mobile station is given by the intersection of the hyperbolas defined by the GTDs, as shown in FIG. 2. Accordingly, at least two GTD values are required in order to locate the mobile station.
Once a mobile station has measured the OTD values, in order to obtain the desired GTD values, the RTDs must be known. In addition, in order to obtain the location of the mobile station from the GTDs, the location of the base stations must be known. There are several methods for obtaining the RTD and the base station location.
One method of obtaining the RTD values is to measure the real time differences using Location Measurement Units (LMUs). In its simplest form, an E-OTD LMU consists of an E-OTD capable mobile station placed at a fixed known location in the network. The E-OTD LMU measures the OTDs between pairs of base stations, and assuming that the locations of the LMUs and the base stations are known (i.e., the GTDs are known), RTD values can be calculated using the following equation:RTD=OTD−GTD  (6)Depending on the network and the location of the LMUs (i.e., co-located at a base station site or at a location between base stations), the ratio between base stations and LMUs in the network is estimated to be 3:1 (i.e., for every 3 base stations in the network one LMU is required). Drawbacks of the LMU approach are that it requires new hardware to be deployed in the network, and depending on the stability of the base station clocks, the LMU's may have to report RTDs quite often, which increases the network load.
An alternative method of determining the RTD values is to synchronize the network. This is the case, for example, in IS95 where GPS is used as a timing source. Also for GSM and TDMA, synchronization is being discussed and is indeed a requirement for deployment of a technology called compact EDGE. In a synchronized network, the base stations (or LMUs) don't need to report the RTDs since they are assumed to be constant. However, the synchronization is usually obtained in the digital baseband domain and not the air interface. It will be recognized that synchronization in the air interface is important for accurately determining location as a number of additional delay sources are present in the air interface which must be accounted for, including transmit filters, combiners and transmission lines. One solution for obtaining air interface synchronization is to use an LMU which measures the transmission on the air interface from its serving cell and relates the base station time to absolute time. The alternative of synchronizing the network partially overcomes the drawbacks associated with the LMU method, however; the synchronization needs to take the air interface into account. The required level of synchronization on the air is 5 μs for IS95, which is enough for communication, but far too lax for location services (5 μs corresponds to a traveled distance of 1500 m). Factory calibration of all delay sources, including transmit filters, combiners and transmission lines is one possible solution to accounting for the air interface, but this may be prohibitive for cost reasons.
The base station and LMU locations are normally not known precisely by the operator today, or are not accurate to the level required by location systems. This is due to the fact that for communication purposes exact locations are not that critical. To deploy location systems, therefore, it is necessary to measure precisely the BS and LMU locations, possibly by maintenance personnel equipped with differential GPS receivers.
The problems with base station location calibration is that maintenance personnel need to visit each site and be equipped with differential GPS receivers. This may be costly and furthermore it may not be possible to obtain GPS coverage everywhere, e.g., inside buildings. In some regions in the world, differential GPS might not even be available. In these regions, the operator will have to rely on the accuracy provided by the standard service mode of GPS, which gives location accuracy of around 100 m, 95% of the time.
Accordingly, the invention provides a method for calibration of a positioning system within a telecommunications network which overcomes the drawbacks associated with known calibration methods. The method of the invention has the advantage that no new hardware is required, as it is entirely based on measurements which are made by multiple mobile stations.